The present invention relates generally to devices for fixation of parts of a to fractured bone and, more specifically, to bone plates and systems for fixation of parts of a fractured bone.
Bone plates are commonly used for treatment of a fractured bone. For instance, bone plates may be used in conjunction with bone screws to stabilize parts of a fractured bone. The bone plate may have threaded holes for engaging a threaded screw-head to provide an angularly and axially stable connection between the plate and bone. In this configuration, a certain minimum thickness of the plate is needed in order to provide a secure connection between the bone plate and the screw bead. Otherwise, the connection may lack axial and angular stability. Traditionally, a thick plate has been required to achieve the desired stability of the screw-plate interface. However, it is often not practical for bone plates to be of the required thickness because they may not provide desirable properties, such as flexibility or adaptability. Thus, a need exists for bone plates that are relatively thin, but have holes that provide a stable connection between the plate and screw.
The present invention is directed to a bone plate having a plurality of holes for receiving bone screws. The bone plate may have an increased thickness in a region surrounding at least one of the holes. This combination of thicknesses is made possible by providing a protrusion formed on one side of the plate that at least partially surrounds the hole. The hole may be provided with a thread or other structure configured and dimensioned to engage a screw-head.
According to one embodiment of the invention, the plate may be thickened by adding a tapered flange protruding from the bone contacting surface of the plate. A corresponding tapered indentation may be provided on the upper surface of the plate, opposite the flange. The flange and/or recess may be formed on the plate, for example, by drawing the plate in the area around the hole.
The protrusion may define an increased plate thickness of the bone plate in the region immediately surrounding the hole, and the bone plate may define a nominal plate thickness is regions spaced from the hole. The increased thickness may be about 1.1 to about 4 times the nominal thickness. Preferably, the increased thickness is about 1.5 to about 2 times the nominal thickness. The protrusion may protrude from the bone contacting surface by about 0.1 to about 3 mm, and preferably by about 0.5 to about 1 mm. The nominal plate thickness may be about 0.5 to about 2 mm, and preferably about 0.8 to about 1.2 mm. In the case where the protrusion is a conical flange, the flange may define an outer conical angle of between about 5xc2x0 and about 120xc2x0, and preferably of between about 40xc2x0 and about 100xc2x0.
As mentioned above, the hole may be provided with at least one internal thread for engaging a portion of a bone screw. The thread may be xe2x80x9cstraightxe2x80x9d provided on the inner surface of a substantially cylindrical hole) or xe2x80x9ctaperedxe2x80x9d (provided on the inner surface of a hole that is tapered or angled with respect to its central axis). One of ordinary skill in the art will know and appreciate, however, that the thread may alternatively be provided in a hole having an inner surface profile that is arcuate, free-form or otherwise shaped and dimensioned. In the case of a tapered thread, the thread preferably tapers radially inward toward the bone-contacting surface. Tapered threads may have a taper angle (indicated as number 13 in FIG. 1) of about 5xc2x0 to about 50xc2x0, and preferably of about 10xc2x0 to about 30xc2x0. According to the exemplary embodiment shown in FIG. 1, taper angle 13 is about 20xc2x0. One of ordinary skill in the art will know and appreciate that taper angle 13 may vary along the axis of the hole.
The threads, if provided, may be either single threads or double threads. Single threads may have a pitch of about 0.4 mm to about 1.5 mm, and preferably of about 0.60 mm to about 1.25 mm. Double threads may have a pitch of about 0.2 mm to about 1.5 mm, and preferably of about 0.3 mm to about 0.8 mm. One of ordinary skill in the art will know and appreciate, however, that different pitches may be appropriate for different applications of the bone plate.
According to one embodiment, the axis of the holes may be oriented perpendicular or substantially perpendicular to the upper surface of the plate. In the case where multiple holes are provided, the axes of the holes may be substantially parallel to one another. It should be noted however, that the axes of the holes may alternatively be angled with respect to the plate, and/or to each other.
The present invention is also directed to a bone plate system including at least one bone screw. The bone screw preferably has a thread disposed on its screw-head. In the case where the bone plate is provided with one or more holes having tapered threads, the bone screws to be inserted in those holes are preferably provided with screw-heads having tapered threads as well (i.e., the threads are provided on a tapered screw-head), in which case, the taper angle of the screw-head preferably matches the taper angle of the screw hole. This configuration provides an enlarged engagement surface between the screw-head and the hole, thus allowing more threads to be formed on the hole; ultimately providing improved anchoring of the screw to the bone plate, while maintaining a relatively thin bone plate. This provides advantages such as increased strength of the plate-screw interface, while maintaining the desired mechanical properties of the bone plate, such as flexibility and adaptability to the bone. Thus, the bone plate of the present invention may be advantageously used in applications where a thin bone plate is desirable, such as, for example, with the spinal column, pelvis, or with tubular bones.